The requirements of drug dose and dosing frequency are unique to pregnancy, in that the renal clearance is altered due to physiologic changes in the maternal renal system, and the addition of the utero-fetal compartment. In addition, the dose frequency required during the third trimester has never been studied. Penicillin is cleared exclusively by the renal system, and during the third trimester of pregnancy, the renal blood flow and renal filtration rates are many times that found outside pregnancy. The current dosing recommendations for GBS prophylaxis by the CDC are not based on physiologic parameters found during pregnancy. Rather, the dosing is based on data from non-pregnant males, from experiments conducted during the 1950's. We therefore propose to determine the proper dose and dosing frequency during the third trimester of pregnancy. We further hypothesize that if the proper dose and frequency of penicillin is used, this will thereby decrease the rate at which resistance to penicillin by Group B Streptococcus will develop over time. With the uncontrolled use of penicillin, an increased frequency of drug resistance for GBS may result. This in turn could lead to higher infant morbidity and mortality from multi-drug resistant GBS.